Graduated sige alloy thermocouple

ABSTRACT

METHOD AND APPARATUS PROVIDING A MECHANICALLY SOUND, LOW ELECTRIC AND THERMAL RESISTANCE, METAL CONTACT FOR COLD JUNCTIONS FOR HIGH SILICON CONTENT SILICON-GERMANIUM ALLOYS FOR USE IN THERMOCOUPLES FOR THERMOELECTRIC POWDER GENERATION.

July 2, 1974 F. KOT 3,822,152

GRADUATED SIGE ALLOY THERMOCOUPLE Original Filed March 30, 1971INVENTOR.

F rclncis Koi 487: /?M

United States Patent 01 fice 3,822,152 Patented July 2, 1974 3,822,152GRADUATED SiGe ALLOY THERMOCOUPLE Francis Kot, Harrison, N.J., assignorto the United States of America as represented by the United StatesAtomic Energy Commission Continuation of abandoned application Ser. No.101,539, Mar. 30, 1971. This application Apr. 25, 1973, Ser. N 0.354,415

Int. Cl. H01v 1/08 US. Cl. 136--237 2 Claims ABSTRACT OF THE DISCLOSUREMethod and apparatus providing a mechanically sound, low electrical andthermal resistance, metal contact for cold junctions for high siliconcontent silicon-germanium alloys for use in thermocouples forthermoelectric power generation.

CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation ofapplication Ser. No. 101,539, filed Mar. 30, 1971 now abandoned.

Co-pending US. Application S.N. 841,958, filed July 15, 1969; by ArthurJ. Seuifert on Method for Producing a Chromium-Tungsten Coating onTungsten for Protection Against Oxidation at Elevated Temperatures, as-Signed to the assignee of this application.

Co-pending US. Application S.N. 697,281, filed J an. 5, 1968; by JamesE. Monroe on Technique for Applying Thermal Shock Resistant andObtaining Variable Emissivity Coatings for Metal Structures, assigned tothe assignee of this application.

BACKGROUND OF THE INVENTION This invention was made in the course of, orunder a contract with the United States Atomic Energy Commission.

In the field of nuclear auxiliary power and/or the field referred tohereinafter by the acronym SNAP, it has often been advantageous toprovide a tungsten cold shoe contact junction between the silicongermanium alloy thermoelectric portion of the cold stack and theremainder thereof, such as described and shown in the above-mentionedcopending application by Seuifert, as well as US. Pat. 3,342,567, and/orU.S. Pat. 3,235,957. As described in the above-mentioned co-pendingapplication, the silicon germanium alloy has comprised 63.5 At. percentsilicon in one attempt to match the thermal expansions of the silicongermanium portion of the cold stack and the tungsten cold shoe contact.However, the success of this attempt has been mixed or unsatisfactory.For example, cracking of the SiGe-W contact junction has been a problem.Likewise, it has been difficult heretofore uniformly and consistently toobtain high electrical and/or thermal contact resistance. It hasadditionally been advantageous to provide high Si content SiGe alloythermoelectric elements and contacts therefor and/or to employ themdependably for use in SNAP applications.

It is an object of this invention, therefore, to provide improvedfunction contacts for SiGe thermoelectric elements;

It is also an object to provide an improved silicon germanium,thermoelectric cold stack;

It is also an object to provide close thermal expansion matches betweensilicon germanium thermoelectric elements and the cold junctionstherefor;

It is an another object to prevent cracking in SiGe-W thermoelectricjunctions;

It is a further object to provide high silicon content SiGe alloythermoelements, and tungsten contacts therefor;

It is a still further object to provide low electrical and thermalthermoelectric junction resistances.

SUMMARY OF THE INVENTION 1 This invention provides a specificthermoelectric device having a specific SiGe element, and a specifictransition piece sandwiched between the mentioned SiGe element and atungsten cold junction contact thereof. As such, this invention is animprovement on the inventions described, in the above-citedapplications, wherein there is provided an improved thermoelectricdevice having a superior tungsten-silicon germanium junction. Moreparticularly, this invention provides a high silicon content SiGe alloythermoelectric element and a specific silicon-germanium transltion piecebetween the SiGe thermoelectric element and a tungsten cold shoejunction. In one embodiment, the SiGe alloy thermoelectric element hastherein at least 70 At. percent Si, and the transition piece is a 63.5At. percent silicon-germanium alloy between the cold shoe and the highsilicon content SiGe alloy. This has the advantage of combiningthermoelectric advantages of the high silicon content SiGe alloythermoelectric element, with the proven superiority of the tungsten coldshoe contact. With the proper selection and assembly of the elements thedesired mechanically sound, low electrical and thermal resistancethermoelectric device and cold junction contact therefor are achieved inaccordance with invention, as described in more detail hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS This invention is useful in formingcold junctions for silicon-germanium thermoelectric devices and moreparticularly to such devices that operate at over 600 C., in vac. forterrestrial applications, and in a vacuum for space applications.However, the method and apparatus of this invention are useful in any ofthe applications to which the subject matter of the above-mentionedcopending applications are useful, as will be understood by one skilledin the art from the following.

Referring now to the single figure, thermoelectric device 11 forms astack 13 of elements comprising a SiGe alloy thermoelectric element 15having 70 to 85 At. percent Si for operation in a terrestrial primarylaunch ambient and a low pressure space ambient for the production ofelectrical power due to a thermal gradient across the element 15 from ahot junction end 17 to a cold junction end 19 thereof. Advantageously,to this end of a plurality of these thermoelectric devices 11 areassembled in a cascade such as disclosed by the above-mentionedco-pending applications to form a converter having a heat source, suchas the radiosotope heat source contemplated by said application, as willbe understood therefrom and from US. Pat. 3,192,069 or US. Pat.3,520,734 but which is not shown herein for ease of explanation. It willbe understood that in one embodiment, the SiGe element15 of the device11 shown in the single figure, advantageously comprises atomic percentsilicon, and a remainder comprising about 20 atomic percent germaniumhaving small amounts of other materials therein, such as conventional nor p type dopants that since they are well known in the art, are notdiscussed herein in more detail. Advantageously, the SiGe element 15 ishot-pressed and zoned leveled by conventional means well known in theart, as discussed e.g. in reports ALO(2510)-1 through 10 under RCAcontract AT(29-2)2510 with the U.S.A.E.C. In this regard, thesethermoelectric elements are adapted to be operated at hot junctiontemperatures of up to 1100 C. or more and cold junction temperatures inthe vicinity of 500 C.

In one embodiment, the parameters of elements 14 of one device 11, areprovided in the following and Table I:

TABLE I Insulation Metallic Foil & molybdenum and/or zirconia cloth.Thermoelectric Device 11 Hot shoe 23 1.0" x 1.0" x 0.75", HotpressedSiMo.

Element 2.03 cm. (0.8") long.

n-Type Element 0.1935 cm. (0.03 in?) area.

p-Type Element 0.1935 cm. 0.03 in?) area, Hot pressed 80% SiGe.

Leg diameter .275".

No. of Devices 11 18.

Spacing between Hot Shoes 23 0.070".

Cold Stack Gold components.

Radiator 33 Beryllium having inconel 750x stud attachment.

Tungsten body 35 has a desired disc-shaped form and size, and opposingmajor faces formed with a diameter of about 0.5 inch and a thickness ofabout .02 to .04 inch. One such W body is described in US. Pat.3,342,567.

Heretofore, the bonding of such tungsten bodies 35 to SiGethermoelectric elements 15, and as those described above, has beendifficult or has resulted in defects such as cracks and the like. Also,the junctions 37 between the heretofore known W-SiGe elements 14 haveoften had undesirably high thermal and/or electrical resistances.However, should a specific At. percent Si containing SiGe cold shoe disc41 be sandwiched in between W body 35 and 80 At. percent Si containingSiGe element 15 so as to contact therewith in accordance with thisinvention, those heretofore known problems can be avoided.

The above-described specific cold shoe disc 41 has the advantage of aclose thermal expansion match with the above-described W body 35. Thisprevents cracking in the SiGe-W junctions 37 and provides a desired lowelectrical and thermal resistance across junctions 37. Additionally, thedescribed specific combination has the advantage of improved efiiciencyand long-term stability at elevated temperatures. In this regard, thedescribed element 15, which comprises 80 At. percent Si, is superior inthe mentioned particulars to elements 15 having higher or lowerpercentages of Si.

Also attempts to bond tungsten bodies 35 directly to the describedelements 15, have heretofore resulted in a high incidence ofunsatisfactory bonds due to the mismatch elements. However, the use ofthe described specific cold shoe disc 41 between the described W body 35and the specifichigli Si element 15 in accordance with this invention,avoids this heretofore known bonding problem. 5 The method and apparatusof the described invention combine the thermoelectric advantages of thedescribed high silicon content SiGe alloy with the superiority of thedescribed tungsten cold shoe 35, and form an improved thermoelectricdevice 11 for use in a wide Variety of SNAP applications. One suchapplication that this invention is particularly well suited for is thedescribed high temperature space application having radio-isotope heatsource.

What is claimed is:

1. In a SiGe thermoelectric device having a tungsten body, a hot shoeend, and SiGe thermoelectric means between said tungsten body and saidhot shoe end, the improvement consisting of:

a. an 80 At. percent Si content SiGe alloy thermoelectric first elementnext to said hot shoe end; and

b. a 63.5 At. percent Si content SiGe disc-shaped transition elementsandwiched between the 80 At. percent Si content SiGe alloythermoelectric first element and said tungsten body, which isdisc-shaped, so as to provide a graduated SiGe thermoelectric device ina stack.

2. The device of claim 1 in which said graduated SiGe thermoelectricmeans in a stack consists of:

a. said hot shoe end;

b. a hot-pressed and zone leveled, longitudinally extending, 80 At.percent Silicon content, SiGe alloy, thermoelectrical first element atsaid hot shoe end; and

c. a 63.5 At. percent silicon content transition piece forming a coldshoe disc in content with the first element at one end and said tungstenbody at the other end thereof.

References Cited UNITED STATES PATENTS 3,432,365 3/1969 Katz et al.136237 3,496,027 2/1970 Dingwall et a1. 136239 X 3,442,718 5/1969Dingwall et a1. 1'36-239 X 3,728,160 4/1973 DesChamps et al. 136-202.3,050,574 8/1962 Rosi 29-573 UX 3,342,567 9/1967 Dingwall 136-439 X3,452,423 7/1969 Webb 136--205 3,666,566 5/1972 Paine 136-202 OTHERREFERENCES U.S.A.E.C., High-Temperature Radiosotope Thermo electricGenerator for Space Applications, Report TID-22350, available to publicNov. 15, 1965, pp. 63-

E. A. MILL-ER, Assistant Examiner US. Cl. X.R. 136202, 239

of the thermal expansion coetficients of these described

